Metal micromachined

Nordquist CD, Wanke MC, Rowen AM, Arrington CL, Grine AD, Fuller CT (2011) Properties of surface metal micromachined rectangular waveguide operating near 3 THz. IEEE J Sel Top Quantum Electron 17(1) 130-137... 

Nanotechnology involves the manipulation of matter on atomic and molecular scales. This technology combines nanosized materials in order to create entirely new products ranging from computers to micromachines and includes even the quantum level operation of materials. The structural control of materials on the nanometer scale can lead to the realization of new material characteristics that are totally different from those realized by conventional methods, and it is expected to result in technological innovations in a variety of materials including metals, semiconductors, ceramics, and organic materials. 

CO Resistive sensors pellistors, metal-oxide sensors Optical sensors micro-spectrometer, IR-sources, IR-detectors, IR-filters Hybrid or integrated, surface micromachining Sn02 sintered thick film (Figaro, FIS,. ..), Sn02 thin and thick film on silicon (MiCS, Microsens) IR spectroscopy (Vaisala, Honeywell,. ..)... 

The third block in Fig. 2.1 shows the various possible sensing modes. The basic operation mode of a micromachined metal-oxide sensor is the measurement of the resistance or impedance [69] of the sensitive layer at constant temperature. A well-known problem of metal-oxide-based sensors is their lack of selectivity. Additional information on the interaction of analyte and sensitive layer may lead to better gas discrimination. Micromachined sensors exhibit a low thermal time constant, which can be used to advantage by applying temperature-modulation techniques. The gas/oxide interaction characteristics and dynamics are observable in the measured sensor resistance. Various temperature modulation methods have been explored. The first method relies on a train of rectangular temperature pulses at variable temperature step heights [70-72]. This method was further developed to find optimized modulation curves [73]. Sinusoidal temperature modulation also has been applied, and the data were evaluated by Fourier transformation [75]. Another idea included the simultaneous measurement of the resistive and calorimetric microhotplate response by additionally monitoring the change in the heater resistance upon gas exposure [74-76]. 

A cross-sectional schematic of a monolithic gas sensor system featuring a microhotplate is shown in Fig. 2.2. Its fabrication relies on an industrial CMOS-process with subsequent micromachining steps. Diverse thin-film layers, which can be used for electrical insulation and passivation, are available in the CMOS-process. They are denoted dielectric layers and include several silicon-oxide layers such as the thermal field oxide, the contact oxide and the intermetal oxide as well as a silicon-nitride layer that serves as passivation. All these materials exhibit a characteristically low thermal conductivity, so that a membrane, which consists of only the dielectric layers, provides excellent thermal insulation between the bulk-silicon chip and a heated area. The heated area features a resistive heater, a temperature sensor, and the electrodes that contact the deposited sensitive metal oxide. An additional temperature sensor is integrated close to the circuitry on the bulk chip to monitor the overall chip temperature. The membrane is released by etching away the silicon underneath the dielectric layers. Depending on the micromachining procedure, it is possible to leave a silicon island underneath the heated area. Such an island can serve as a heat spreader and also mechanically stabihzes the membrane. The fabrication process will be explained in more detail in Chap 4. 

The central topic of the book was the integration of microhotplate-based metal-oxide gas sensors with the associated circuitry to arrive at single-chip systems. Innovative microhotplate designs, dedicated post-CMOS micromachining steps, and novel system architectures have been developed to reach this goal. The book includes a multitude of building blocks for an application-specific sensor system design based on a modular approach. 

I. Simon, N. Barsan, M. Bauer, and U. Weimar. Micromachined metal oxide gas sensors opportunities to improve sensor performance . Sensors and Actuators B73 (2001), 1-26. 

In the previous section, it was mentioned that Zhang et al. [51,52] developed a technique to micromachine a thin metal film made out of copper. This material had a number of perforations that followed a predetermined pattern and such a design can be changed fairly easily by simply changing one of the masks used in the fabrication process. Although copper is not an ideal material to be used as a DL in a fuel cell due to the contamination issues, the development of fabrication techniques similar to those mentioned... 

K. Fushinobu, D. Takahashi, and K. Okazaki. Micromachined metallic thin films for the gas diffusion layer of PEFCs. Journal of Power Sources 158 (2006) 1240-1245. 

Hollow planar waveguides have been fabricated by several techniques, including physical vapor deposition and CVD of silver and dielectric layers on metallic substrates. Nevertheless, better results can be obtained by taking advantage of silicon micromachining techniques. Perhaps the most important advantage of silicon hollow waveguides over other hollow structures is the... 

Excimer Laser Micromachining [132, 133] is a technique based on laser ablation. Currently, this process can routinely ablate vias as small as 6 pm in diameter in polymers, glass, ceramics and metals. The minimum size of the features that this method can produce is limited by diffraction and by heat/mass transport. Commercial instruments and services are available from a number of companies (for example, Resonetics, Itek). 

Electrochemical micromachining (EMM) is a technique designed to generate patterned microstructures in metals and alloys [ 145]. Microfabrication by EMM may involve maskless or through-mask dissolution. Maskless EMM uses the... 

Micro electrode arrays can also be produced by thin film technology and silicon micromachining. Electrochemical analysis using planar thin film metal electrodes as transducer can be done with high performance in vitro [59]. 

Metal layers have been deposited on microchip substrates. The metal layer was either used as an etch mask in micromachining, or used for detection (e.g., as metal electrodes for electrochemical detection). Various metals have been used as the overlayer and adhesion layer, as summarized in Table 2.10. 

Figure 1.3 Natural diamond cutter for micromachining of metals.

Substrates The substrates in microelectronics are mainly Si wafers. For mobile applications, silicon-on-insulator (SOI) wafers increasingly replace bulk Si wafers and for very specific high-frequency applications, III-V compound semiconductors (e.g., GaAs) are used. The majority of substrates in microfabrication are Si wafers, but metal, glass, and ceramic substrates are also common. Particularly when using glass, quartz, and ceramic wafers in CMP processes, it has to be taken into account that they are brittle and easy to break. The situation is worse when the material is also under stress induced by deposited layers. For applications where the backside of the wafer has to be structured (e.g., in bulk micromachining), double-side polished substrates are employed. 

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